The present invention relates to a method of controlling the speed reduction ratio of a continuously variable speed transmission for use in a motor vehicle.
One known method of controlling the speed reduction ratio of a continuously variable speed transmission mounted on a motor vehicle comprises the steps of establishing a reference engine speed corresponding to a throttle valve opening of the engine, and controlling the engine rotational speed so as to be equal to the reference engine speed.
For controlling the speed reduction ratio in the above manner, it is necessary to optimize the rate of change of the speed reduction ratio for preventing a delay in speed change, racing of the engine, hunting, and the like. To meet such a requirement, there has been proposed a method of controlling the speed reduction ratio of a continuously variable speed transmission by calculating the rate of change of the speed reduction ratio as the sum of a component corresponding to a predicted acceleration and a component corresponding to the rate of change of a reference engine speed, and using the calculated ratio varying speed as a controlling value (see Japanese Laid-Open Patent Publication Nos. 63(1988)-53343 and 63(1988)-53344, for example).
With the proposed method, when the actual engine speed is lower than the reference engine speed, in order to increase the actual engine speed up to the reference engine speed, a rate of change (which is the component corresponding to the rate of change of the reference engine speed) of the speed reduction ratio corresponding to the difference between the actual and reference engine speeds is calculated, and the speed reduction ratio is controlled on the basis of the calculated rate of change.
The rate of change of the speed reduction ratio is usually calculated by a microcomputer or the like at prescribed time intervals (each of several tens of milliseconds, for example). After a rate of change of the speed reduction ratio has been calculated according to the engine speed and the throttle valve opening at a certain time, the calculated rate of change is maintained until a next rate of change of the speed reduction ratio is calculated upon elapse of the above time interval. During this time interval, the actual engine speed may become higher than or overshoot the reference engine speed.
The above problem may be solved by establishing a different rate of change of the speed reduction ratio for the difference between a reference engine speed and an actual engine speed or by lowering the response of the control system. However, it would be quite difficult to establish such a different rate of change of the speed reduction ratio. Individual speed reduction ratio control mechanisms tend to respond differently to the established rate of change, and hence tend to perform different control operations.